@townshend-audio,
One of us has the required education, and experience not to guess at this topic. I don’t need to guess or consult "All About Circuits", though it is for the most part a well designed website.
And yes, I can explain them and I already did. INDUCTANCE. This is well known, and documented by professional wire companies like Kimber and Cardas. (Though to be completely accurate, there may be a fraction of a db here and there for skin resistance).
Everything in your article points to 1 and only one 1 item. Inductance. Not characteristic impedance. Plain, simple inductance. Space conductors far apart, and the inductance is high. Space conductors close together and the inductance is low. Put two flat conductors really close and the inductance is very low (and the capacitance very high which can make some amplifiers unhappy).
Let me point out that your statement, "The results, Fig 3, show the frequency responses of a series of cables from 30Hz to 20kHz, together with their characteristic impedances Z0." is wrong. It does not show the frequency response, it shows the cable voltage drop, which is not the same as frequency response.
As we don’t know what drive level the amplifier is, the spectrum analyzer settings, or even the scale, though it may be in db, but was that dbW, dbm, dbu, dbuv? db without anything else is a relative number only when measured electrically. Again, not a frequency response, a relative voltage drop.
Looking at your round conductors, spaced at 5, 15, 50mm. Depending on the gauge, the ratio of inductance will be close to 1:2:3, with 1 being between about 6 an 20uh, but the construction and dielectric differences in your samples will make for a lot of variation, but first order calcs would show about 5db difference between 5mm and 15mm and a bit less between 15 and 50mm, which is not too far off the 5 or 6db differences you show.
The parallel flat plates of your cables (and Alpha Core Geortz cables), I would estimate as only about 3-5uH. I would need more details on the dimensions, thickness of the dielectric, etc., but rough is going to be 3-5uH. That would be 1/2 the best case close wires, and maybe 1/3 - 1/4 and hence why less voltage drop at 20KHz. .... oops see it is 6.6uH in your document. My 5uh upper end was not a bad estimate. No idea what frequency your LCR measures at though, so that 6.6uH may not be accurate at 20KHz.
No transmission lines, no reflections, just basic physics / electrical engineering. You may notice that as opposed to talking in generalities, I have, in both my posts, brought up very specific numbers. Those weak on a topic guess, those who understand take available information and develop relatively accurate estimates.
Oh, FYI, you show the cable drop as lower at 20KHz than 0Hz so something in your system is dropping 2.1db at 20KHz.
